1. Field of the Invention
The present invention relates to a non-volatile memory device adapted for use in a computer, a memory card, a word processor, and the like. More particularly, the present invention relates to a non-volatile memory device having a large, highly-dense capacity and is capable of electrical writing and reading information.
2. Description of the Prior Art
As non-volatile memory devices, the following four devices are well known and widely used:
(1) magnetic tapes; PA1 (2) magnetic disks; PA1 (3) IC non-volatile memory such as EPROM and EEPROM; and PA1 (4) optical magnetic disks. PA1 (a) A large, dense storage capacity can be achieved; PA1 (b) Anti-shock, anti-vibration can be ensured; PA1 (c) The size can be compact and simple, and the price must be inexpensive; and PA1 (d) A high speed reading and writing are achievable.
Each of these memory devices will be more particularly described:
(1) Magnetic Tapes:
This is a typical data replaceable non-volatile memory device, and is in wide use for audio tapes and video tapes because of its inexpensive price. In addition, because of its large capacity, it is used for back-up memory in a computer. The disadvantages of the magnetic tape are that the function it can perform is only the writing and reading of information and the access time is long in writing and reading.
(2) Magnetic Disks:
Magnetic disks are in wide use as an external memory device for computers and word processors. Magnetic disks include floppy disks which are handy and inexpensive, and hard disks which have a large storage capacity but are difficult to handle and are expensive. The advantages are that high speed random access is possible, and writing and reading are readily practicable. A 3.5 inch floppy disk has a storage capacity of about 1 megabytes (M bytes), and a 3.5 inch hard disk has a capacity of about 40M bytes. However, the capacity and density cannot be increased further.
(3) EPROM and EEPROM:
Each is a typical example of IC non-volatile memory devices which allow the ready replacement of data and highly dense storage. There are two types; an EPROM in which the writing is electrically effected, and erasing is effected by ultraviolet irradiation, and an EEPROM in which the writing and erasing are electrically effected. Advantageously, this type of IC non-volatile memory device has a small and lightweight size, has a short access time, and is energy-saving.
Referring to FIG. 9, the writing and erasing under the EEPROM system will be described:
Information is recorded by applying voltage between a drain area 6 and a control gate 2 and injecting a carrier into a floating gate 4. The recorded information is erased by applying voltage between the source area 8 and the control gate 2, and removing the carrier under the Fowler-Nordheim (N-F) Tunneling phenomenon. The recorded information is read by judging the ON or OFF by a threshold voltage of a reversal voltage. Since the carrier is injected and removed through a gate oxide layer 5, the quality and thickness of the carrier affect the quality of EEPROM. For example, an EEPROM having a storage capacity of 1 Mega bit (M bit) has a gate oxide layer having a thickness of 20 nm. It is difficult to produce layers to such a narrow thickness, thereby decreasing the yield of production. As a result, the production cost is increased. A chip normally has sides of 7 to 10 mm in length, and in order to increase the storage capacity, the area of a chip is increased but this reduces the production yield. As a result, the production cost is increased.
Because of these disadvantages, the storage capacity of an EEPROM cannot be increased beyond a certain limit. In recent years, the average storage capacity of EEPROMs is in the range of 1 to 4M bits, which is smaller than that of magnetic disks and optical magnetic disks.
(4) Optical Magnetic Disks:
This is a typical example of a non-volatile large capacity data replaceable optical disk.
Referring to FIG. 10, this type of disk utilizes a laser beam 20 so as to record data, and the advantage is that since the data is recorded through a transparent glass substrate 12 in a non-contact state, dust present on a recording surface 23 is not likely to prevent the recording of data. Since the laser beam is not focused on the substrate 22, and has a larger diameter of about a few hundreds of microns, the presence of dust will not seriously affect the recording of data. Optical-electromagnetic disks have a relatively large storage capacity because the focused laser beam 20 records and reproduces, and therefore, a highly dense recording is achieved. For example, a 3.5 inch disk has a storage capacity of about 120M bytes.
One of the disadvantages is that a rotary device is required for rotating the laser generator, the magnet, and the disk for writing and reading the information, thereby requiring a relatively large peripheral device and increasing the production cost.
For an ideal type of non-volatile memory device, the following requirements (a) to (d) must be satisfied:
The known memory devices listed (a) to (d) above do not satisfy all these requirements.